In the semiconductor manufacturing industry, plasma etching is widely used in wafer processing. It is typically used to create features such as contact holes or trenches in materials in order to form electrical contacts. Given that space is an important consideration in manufacturing, it is often desirable to make these contact holes as narrow and as deep as possible. The etching of such narrow and deep features poses many challenges due to various complications and difficulties that arise during the plasma etching process.
FIG. 1 illustrates a cross-sectional view of a left side of a conventional wafer processing system during a conventional plasma etching process. Wafer processing system 100 includes a processing chamber 102, an upper electrode 104; an electro-static chuck (ESC) 106 and an RF driver 110. Processing chamber 102, upper electrode 104 and ESC 106 are arranged to provide a plasma-forming space 108. RF driver 110 is electrically connected to ESC 106, while upper electrode 104 is electrically connected to ground.
In operation, a wafer 114 is held on ESC 106 via an electrostatic force. A gas source (not shown) supplies an etching gas to plasma-forming space 108. RF driver 110 provides a driving signal to ESC 106, thus providing a voltage differential between ESC 106 and upper electrode 104. The voltage differential creates an electromagnetic field in plasma-forming space 108, wherein the gas in plasma-forming space 108 is ionized, forming plasma 112. Plasma 112 etches the surface of wafer 114.
An example conventional process of etching a wafer will now be discussed below with reference to FIG. 2A and FIG. 2B.
FIG. 2A illustrates a cross-section of wafer 114 as shown in FIG. 1, before plasma 112 is formed and thus before any material on wafer 114 is etched. In FIG. 2A, wafer 114 includes a substrate 200 and a mask 202. Mask 202 serves to define the areas of substrate 200 that are to be etched by plasma 112. The portion of substrate 200 that is not covered by mask 202 (unmasked area 204) is exposed to plasma 112 and therefore will be etched away during the etching process. Conversely, the portions of substrate 200 that are covered by mask 202 (masked areas 206) are not subjected to plasma 112, and therefore will not be etched away during the etching process. Mask 202 itself, being exposed to plasma 112, is also etched somewhat; however, the properties of plasma 112 are typically chosen such that the etch rate of mask 202 is much slower than that of substrate 200 (giving it high selectivity), thereby leaving mask 202 mostly intact.
FIG. 2B illustrates a cross-section of wafer 114, after plasma 112 has been formed and the etching process has begun. During the etching process, the surface of wafer 114 is bombarded by incident plasma ions 208 from plasma 112. In unmasked area 204, incident plasma ions 208 etch away a portion of substrate 200, forming an etched hole 212. During the etching process, the incident flux of polymerizing neutral species from plasma along with incident plasma ions 208 causes a polymer layer 210 to be deposited on the exposed wafer surface (mostly on top surface of mask 202). The buildup of polymer layer 210 generally serves to prevent the undesired etching of mask 202, thereby making the etch process more selective towards the material of substrate 200. Alternatively, the incident neutral and ion species can act to reduce the etch rate of mask 202, thereby making the etch process more selective towards the material of substrate 200.
As shown in FIG. 2B, etched contact hole 212 has hole height 214 (noted as h1) and hole diameter 216 (noted as d1). An aspect ratio is defined as the height divided by the diameter. In this case, the aspect ratio of etched hole 212 is defined as h1/d1. Conventionally, contact holes with a relatively low aspect ratio like etched hole 212 can be etched relatively easily with minimal distortion of the hole, as will described in further detail later. However, in several semiconductor applications, there is a high demand to provide high aspect ratio etching, such as to form high aspect ratio contacts (HARC), which involves the etching of very deep holes with small diameters. There are several challenges in the conventional method for providing a HARC etch process, as will now be described with reference to FIGS. 3A-5B.
FIGS. 3A and 3B are graphs representing signals provided by RF driver 110 as a function of time. FIGS. 3C and 3D are each graphs representing ion flux as a function of ion energy, of the signals illustrated in FIGS. 3A and 3B, respectively.
FIG. 3A includes function 300, which is a low-frequency driving signal. FIG. 3B includes function 302, which is a driving signal comprised of a low-frequency portion and a high-frequency portion. FIG. 3C includes function 304, which is the measured ion flux as a function of ion energy that results from using function 300 in FIG. 3A as the driving signal provided by RF driver 110. FIG. 3D includes function 308, which illustrates the predicted ion energy distribution that would result from implementing function 302 of FIG. 3B as the driving signal provided by RF driver 110.
As shown in FIG. 3C, function 304 exhibits a first peak 306 for lower ion energies and a second peak 308 for higher ion energies. As illustrated in the figure, first peak 306 is much larger than second peak 308. Accordingly, lower ion energies as represented by larger first peak 306 will have an effect on process results. For some process requirements, it is considered beneficial to provide a higher flux of low energy ions. In other words, it is beneficial for peak 306 to be as large as possible and at the lowest energy possible. Low energy ions are considered beneficial for two reasons. First, they may reduce feature charging during an etching process by discharging sidewalls. Specifically, because the positive ions have low energy, they are attracted to negatively-charged regions on the feature surface, thereby reducing the feature charging. Second, low energy ions may contribute to polymer deposition during an etching process to protect a mask.
Function 304 is shown in FIG. 3D as a dotted line for reference. As shown in FIG. 3D, function 310 contains a first peak 312, which is shifted to a higher ion energy from peak 306 of function 304. Further, function 310 contains a second peak 314, which is shifted to a lower ion energy from peak 308 of function 304. Similarly, as discussed with reference to FIG. 3C, lower ion energies as represented by first peak 312 will have a significant effect on process results.
The addition of a high-frequency portion in the driving signal as illustrated in FIG. 3B provides an increase in plasma density. As such, the amount of ion flux corresponding to ion energy of first peak 312 of FIG. 3D is greater than the amount of ion flux corresponding to ion energy of first peak 306 of FIG. 3C. Therefore, it is clear that the introduction of a high-frequency portion in the driving signal (switching from function 300 to 302) shifts the ion energy distribution and provides an overall increase in plasma density and ion flux.
FIG. 4 is a graph illustrating the plasma sheath potential at wafer 114 in a conventional method for providing a HARC etch in which function 302 of FIG. 3B is implemented as the driving signal supplied by RF driver 110. In the graph, the x-axis is time, in seconds, whereas the y-axis is the plasma sheath potential, in volts, at the wafer. The plasma sheath potential as a function of time (function 400) is related to the signal provided by RF driver 110. In this example, the signal provided by RF driver 110 includes a superposition of a continuous low frequency portion and a continuous high frequency portion, as shown in FIG. 3B. Therefore, as shown in FIG. 4, the resulting plasma sheath potential (function 400) also includes a superposition of a continuous low frequency portion and a continuous high frequency portion, with some distortion as typically observed for RF plasma sheaths.
Conventional HARC etching processes may use a combination of continuous high frequency and continuous low frequency signals as applied by RF driver 110. Continuous high frequency signals are used to produce high plasma density and, therefore, high ion flux. Continuous low frequency signals are used to produce high plasma sheath potential and, therefore, high ion bombardment energies, as part of a wide distribution of ion energies.
In conventional HARC etching processes. RF driver 110 may provide a driving signal to ESC 110 that includes a superposition of a continuous high frequency portion at a first power and a continuous low frequency portion at a second power (such as function 302 in FIG. 3B). By adjusting the ratio between the power of the continuous high frequency portion and the power of the continuous low frequency portion, one can adjust various plasma properties and can thus adjust the etching characteristics of the plasma. For example, if the power of the continuous high frequency portion of the driving signal is relatively large and the power of the continuous low frequency portion of the driving signal is relatively small, the resulting plasma will be characterized by higher plasma density and ion flux, in combination with lower maximum ion energy. Also, increasing the power of the continuous high frequency portion of the driving signal may increase the polymerization process, thereby resulting in higher contact-to-mask etch selectivity, but can also lead to the etch stop. Additionally, a relatively large power of the continuous high frequency portion of the driving signal will typically increase distortion of the etched hole, as will be discussed below with reference to FIG. 5A.
FIG. 5A illustrates a cross-section of wafer 114 that may result from a conventional HARC etching process. In this example, a driving signal from RF driver 110 includes a continuous high frequency portion and a continuous low frequency portion, wherein the power of the continuous high frequency portion of the driving signal is relatively large and the power of the continuous low frequency portion of the driving signal is relatively small. In this example, the driving signal produces a plasma having a plasma sheath potential described by function 300.
In this example, during the etching process, incident plasma ions 208 in the presence of incident flux of neutral species from plasma 112 bombard the surface of wafer 114. This causes polymer layer 210 to be deposited onto some of the exposed wafer surfaces. At the same time, the bombardment of incident plasma ions 208 causes a portion of substrate 202 in unmasked area 204 to be etched away, forming etched contact hole 500. Etched hole 500 has hole height 502 (denoted as h2) and hole diameter 504 (denoted as d2). Therefore etched hole 500 has an aspect ratio of h2/d2. Here, h2>>h1. Consequently, the aspect ratio of etched hole 500 is notably higher than the aspect ratio of etched hole 212 of FIG. 2B.
However, as shown in FIG. 5A, the walls of etched hole 500 are not completely vertical and the bottom is twisted to one side. Although exact mechanisms are not fully understood, this twisting effect may be explained by accumulated charge on the walls of etched hole 500. FIG. 5B shows a magnified view of the bottom region of etched hole 500, illustrating this accumulated charge in polymer layer 210. The presence of a positive differential charge 506 and a negative differential charge 508 gives rise to an electric field which serves to deflect the downward-directed incident plasma ions 208 towards one side. Since the ion trajectory 510 is now curved towards the right, the etching occurs preferentially towards the right surface instead of at the bottom surface of etched hole 500. This effect therefore causes etched hole 500 to be distorted, or twisted.
In the conventional HARC etch process, the distortion of etched contact holes can be minimized by reducing the power of the continuous high frequency portion of the signal provided by RF driver 110. However, this method decreases the polymerizing properties of the process and therefore decreases contact-to-mask etch selectivity. Also, this method decreases the plasma density and ion flux, thereby slowing down the etch rate.
What is needed is a system and method to provide for the best HARC etch process results with no distortion while at the same time maintaining a high contact-to-mask selectivity and high etch rate for higher throughput.